42 percent and the 6-month yield fell 52 basis points to 7.28 percent at Wednesday's auction, the government's debt office said.
There were no sales of 3-month bills.
Dealers said the rates fell on expectations of a further rate cut at a monetary policy meeting on September 15.
The government rolled over 9.5 billion rupees of bills. Sri Lanka's inflation has been moderate in 2010, though concern has risen about a rise in quantity easing and debt monetization activities by the monetary authority.
The central bank also manages a pension fund of private sector workers, which earlier bought mostly into government securities has been pumping money into stocks.
A Treasury bill stock held by the central bank peaked at 50 billion rupees in August from around 15 to 20 billion rupees in June.
However the central bank also sterilizes liquidity through dollar swaps.
Analysts say the purchase of Treasury bills on one side (printing money) and diversion of pension funds to stocks which would have otherwise gone to fund the fiscal deficit to stocks sets the foundation to fire an asset price bubble.
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